Electrostatic coating application systems for conductive coatings, such as water base paint, for mass production applications are known in the art. In a typical application, a series of work pieces, such as automotive bodies, are conveyed through a paint booth, wherein each work piece may be coated, for example, with a different color paint. In such applications, the coating apparatus typically includes a color changer, a rotary atomizer and a line connecting the color changer and the rotary atomizer, either directly or through a paint cannister. Following each application of coating or paint, the line or lines are generally purged with a solvent, typically ionized water for water base paint. However, similar systems may be utilized for powder coating materials. One means of transmitting the paint or solvent through the lines includes a movable member, such as a pig or pigging element, which is transmitted through the line or lines to either push the coating, separate one coating from another coating, or separate the coating from a solvent, etc. In paint application systems having one or more pigs or pigging elements, it is also conventional to include one or more pig stations. For example, the coating delivery system may, for example, include a first pig station adjacent the color changer and a second pig station adjacent the applicator which is typically a rotary atomizer which imparts a high voltage to the coating. Where the coating delivery system includes an intermediate cannister, the coating delivery system may, for example, include four pig stations, including a first pig station adjacent the color changer, a second pig station adjacent the cannister inlet, a third pig station adjacent the cannister outlet and a fourth pig station adjacent the applicator.
As will be understood, in such coating application systems, it is necessary to determine the location of a movable element or pig in the system under high voltage conditions. For example, a valve may be opened to supply coating material or solvent to the line upon receipt of a pig in a pig receiver station. Typically, the pigs are moved or transferred back and forth between the pig stations. One method of locating a pig in a coating delivery system is to utilize a magnet or other magnetically acting transmitter element in the pig and utilize a magnet sensor in the pig station or adjacent the delivery line. However, where the coating is conductive and subject to high voltage, conventional sensing and actuator systems may not be accurate or require complicated hardware or software as set forth below. Other examples of movable members are the shaft of a metering pump, a rotary atomizer, etc. used in water base paint applications subject to high voltage.
In one disclosed embodiment, the position or movement of which needs to be determined within a section of such a paint application system that is connected to a high voltage, are pigs that can be moved back and forward between pig stations of a color supply line. For example, EP 0 319 172, DE 100 33 987 and DE 101 31 562 describe methods in which a high voltage is applied to the pig stations on the atomizer side during the coating operation of the atomizer via the electrically conductive paint column. Special initiators with pneumatic switches are utilized for determining the pig positions in the lines and pig stations that are connected to a high voltage, wherein said pneumatic switches respond to magnets integrated into the pig body and able to operate under a high voltage. However, a high space requirement and undesirably long reaction times need to be accepted in this case. Similar problems occur in other system sections as described below.
The invention is based on the objective of disclosing a sensor arrangement that makes it possible to determine the position or movement of a moving member within a section of the coating system that is connected to a high voltage without delay and in a space-saving fashion. This objective is attained with the sensor arrangement of this invention.